The present invention relates to a semiconductor laser device having an active layer which is comprised of a quantum well layer made of a mixed crystal material, and barrier layers provided on both sides of the quantum well layer in such a manner as to sandwich the same, and more particularly to improvements of a semiconductor laser device to attain higher efficiency and a shorter wavelength.
Among the wavelength bands which have hitherto been used extensively in semiconductor laser devices, the 800 nm-level band using pure GaAs as an active layer has been dominant. The laser light of this wavelength band, however, has the drawback that the light is utterly invisible to a human eye. In addition, in a case where such a semiconductor laser device is applied as a light source for a photosensitive member of an optical disk or of a laser printer, there has been a demand for attaining a shorter wavelength and rendering the laser light visible to meet requirements for a higher density of the optical disk and to conform with the photosensitive characteristics of the photoreceptor (particularly organic photoreceptor).
In order to attain a shorter wavelength for the output light of a semiconductor laser device, it suffices if the band gap of the light-emitting portion (active layer) is widened, and the following two methods are conceivable as methods therefor. One is a method in which a crystalline material of an active layer is disordered into a mixed crystal (e.g. GaAs .fwdarw.AlGaAs). According to this method, the 700 nm-level laser light can be obtained, and semiconductor laser devices having a wavelength in the vicinity of 780 nm have been made available as products. The other method is one in which the gap between quantization levels contributing to light emission is widened by reducing the thickness of the active layer to the neighborhood of not more than a level (about 200 .ANG.) where the quantum size effect appears (i.e., formation of the quantum well).
Active studies have been undertaken with respect to the latter method since this method is also effective in attaining the high efficiency of the laser light by increasing the effective state density of electrons and holes which contributes to the light emission. If the thickness of the active layer is too small, however, the stability and reliability against the heat treatment of impurity-induced disordering, which will be described later, become deteriorated, so that the thickness which is considered to be practicable is over about 100 .ANG.. Accordingly, in order to attain a sufficiently short wavelength and high efficiency, the joint use of the disordering of the active layer and the formation of the quantum well is desirable, so that the aforementioned active layer is conventionally comprised of a quantum well layer constituted by a mixed crystal material and barrier layers provided on both sides of the quantum well layer in such a manner as to sandwich the same.
Meanwhile, in attaining the higher efficiency of the semiconductor laser device, current restriction and light confinement in the lateral direction in the active layer are required, and various structures as means therefor have been proposed and put to practical use. As an effective means among them, a method using impurity-induced disordering is known. This is a method in which impurities such as Si and Zn are caused to be thermally diffused at a portion other than a light emitting portion from the surface of a crystal-growing layer for a laser device, and mutual diffusion between a quantum well layer at that portion and barrier layers disposed respectively on the upper and lower surfaces thereof is induced so as to perform disordering. The current is restricted in that area by means of this diffusion, and the band gap of the quantum well layers at that portion expands, with the result that the refractive index becomes small and light confinement becomes possible.
A conventional semiconductor laser device will be described hereafter for a case where Si is used as the diffusion impurity. As shown in FIG. 5, an n-type cladding layer c, an n-type barrier layer d, a quantum well layer e of GaAs, a p-type barrier layer d', a p-type cladding layer c', and a p-type cap layer f of GaAs for preventing the oxidation of the p-type cladding layer are consecutively formed as films on an n-type GaAs substrate a. Then, as shown in FIG. 6, Si is diffused in a portion other than a light-emitting portion from a surface of the cap layer until Si sufficiently crosses the quantum well layer e, thereby inducing the disordering between the quantum well layer e and the upper and lower barrier layers d, d'. As a result, since the band gap of the quantum well layer e at this portion expands, the carriers (electrons, holes) cease to be injected into this portion, so that the light-emitting portion g is restricted, thereby contributing to the improvement of the light emission efficiency. At the same time, since the laser light is confined, the high efficiency of the semiconductor laser device is made possible. It should be noted that, as shown in FIG. 7, the conventional semiconductor laser device is arranged such that a p-type electrode h is formed on the p-type cap layer f, and an n-type electrode b is formed on the n-type GaAs substrate a.
However, since heat treatment at high temperature for a long time duration, such as at 850.degree. C. and for 10 hours, is required for the diffusion of the aforementioned Si, in a case where the quantum well layer e is made of mixed crystal of AlGaAs obtained by mixing Al to attain a shorter wavelength, mutual diffusion between the quantum well layer e and the barrier layers d, d' located on and underneath the same is greatly accelerated owing to the aforementioned heat treatment, so that there have been cases where the quantum well layer e and the barrier layers d, d' located on and underneath the same are mixed even at the light-emitting portion g where Si was not diffused. Then, if the quantum well layer e and the barrier layers d, d' are mixed, there have been problems in that the configuration of the quantum well layer changes and the emission efficiency declines, and that the wavelength shifts from a targeted wavelength further toward the short-wavelength side. It should be noted that, in a case where Zn is used instead of the aforementioned Si, heat treatment at high temperature for a long time duration can be made less stringent; however, the diffusion distance of Zn becomes difficult to be controlled, so that its application has been difficult.